Since the 1970's a large portion of gas distribution lines have been fabricated from polyethylene (PE). Also, as of 1995, about one-third of the 1.5 million miles of gas distribution pipelines in this country was made from PE. A special investigative report issued by the NTSB indicates that hundreds of thousands of miles of plastic pipe installed from the 1960's through the early 1980's may be vulnerable to a phenomenon called “brittle-like cracking”. Brittle-like failures, as a national average, may represent the second most frequent failure mode for older plastic piping, exceeded only by excavation damage.
Brittle-like cracking is characterized by the appearance of cracks through the wall of pipe with no visible deformations. This type of failure was not considered during testing in the early years of plastic pipe use; Only hoop stress was considered. When hoop stress was applied to pipe during laboratory testing the usual failure mode was a ductile fracture characterized by significant deformation. The long term hydrostatic strength of polyethylene pipe used for natural gas service was determined by subjecting pipe samples to various hoop stresses and noting the time to failure. A log-log plot of time vs hoop stress was created and then extrapolated to 100,000 hours.
The hoop stress at this point represented the hydrodynamic strength. Only internal pressures were used as design criteria for plastic pipe, external loadings that could cause additional stresses were not considered. It was believed that these stresses would be relieved by local yielding because of the expected ductile behavior of the plastic pipes. After the middle of the 1980's standards were changed and the phenomenon of brittle-like cracking was accounted for. Polyethylene pipe used in new services probably will not exhibit brittle-like cracking but hundreds of thousands of miles of existing pipe may.
A number of techniques exist in literature to determine the integrity of metallic pipelines for gas distribution and gas transmission, such as ultrasonic, eddy current and acoustic wave to name a few. However, no satisfactory technique exists in literature that determines the flaws and defects in non-metallic or plastic pipeline. In particular, no satisfactory techniques exists for the inspection of non-metallic or plastic pipeline in-situ, when significant access constraints and inspection environment concerns may significant logistical hurdles.
To overcome these problems, a suitable sensing device must be able to reliably and dependably detect flaws or abnormalities in plastic and non-metallic pipe.
Herein, we describe a device and method based on capacitance measurements that can detect flaws in already buried plastic gas distribution pipelines. A probe has been designed that can be inserted into the natural gas pipelines to examine the spatial dielectric characteristics of the pipe walls. The probe employs a sensor that measures the capacitance (or permittivity) of the pipeline wall. Defects are indicated by variations in the permittivity of the non-metallic or plastic material.